The present invention relates to a device for sorting pellets in accordance with the preamble of the appended claim 1. The invention is particularly intended for use in connection with sorting of synthetic material pellets, for detecting any defective pellets and for sorting out such defective pellets. The invention also relates to a method for such sorting of pellets in accordance with the preamble of the appended claim 7.
When manufacturing electrical cables for high and medium high tensions, preferably of the order of 50 kV and upwards, one or more electrical conductors are normally provided with a semi-conductive layer and a surrounding insulation coating. In this context it is previously known to use a particular type of oil-cooled paper insulation for the production of this insulation coating. However, in the most recent years, synthetic materials, preferably polythene, have become increasingly common for producing this coating. This choice of material entails certain advantages compared to the previous oil-cooled paper insulation, a/o with regard to maintenance, service and service life. Furthermore, the polythene insulation is a more environment-friendly material than the paper insulation.
Regarding the insulation material for electrical cables, there exists a desire to provide it with as high a temperature resistance as possible. This would in turn warrant, that the insulation coating does not run the risk of melting during any short circuiting currents in such a cable. In connection with medium and high tension cables, it is furthermore of great importance that there is no contamination in the insulating polythene coating, as this could lead to electrical disruption in the cable. Such disruptions can in turn lead to destruction of the cable. When manufacturing the insulating polythene coating, it is however previously known that certain inhomogenities, i.e. contamination, may occur in the material. Such defects could spread with time in the insulation material and cause electrical disruption in a cable.
For the above reasons, there are very stringent demands in the manufacture of cables for medium and high tension, on the purity of the raw material being used for the insulating polytene coating of the cable. This raw material normally consists of pellets, i.e. balls or grains, of polythene. In order to satisfy these stringent purity demands, a fault check is commonly made on this pellet in connection with cable manufacturing. In this manner, those pellets containing any form of defect (i.e. in the form of air bubbles, contamination, deviating symmetry, deviating colour, etc) can be sorted out, as they might otherwise cause the above problems in the finished cable.
According to the known art, there are various methods and devices for sorting pellets. One previously known method is based on using a smaller amount of pellet for manufacturing of a thin film, whereupon this film would be checked through feeding in under a detection device comprising a camera, which in turn detects any defects in the film. If defects are found, the position of these defects can be indicated automatically on the film.
Although this known method generally functions satisfactorily, it has a disadvantage in only being a statistical detection method, by which only a small fraction of the pellets are checked. As the pellet is used to manufacture a film, certain specific pellets cannot be sorted out in this process.
In order to solve the above problem, there is a previously known method in which the raw material, i.e. the pellets themselves, is checked. If all pellets within a manufacturing process could be checked regarding purity, this would create possibilities of purifying the raw material to 100%, which would allow an extremely high purity when manufacturing the insulation coating of an electrical cable.
A previously known arrangement of the last-mentioned type, for sorting the pellet, comprises a conveyor for feeding the pellet and an optical detector (preferably in the form of a CCD camera) that is arranged at a certain position adjacent to the conveyor track and that is connected to a control unit. This control unit is in turn functioning to analyse the signal generated by the detector, in such a way that any existing defective pellets can be sorted out This sorting-out is performed through there being, at the end of the conveyor, a first and a second container. The first container is positioned close to the end portion of the conveyor and the other container is positioned right behind the first container. As long as no defective pellets are detected, all pellets will simply successively fall into the first container, which is thus intended for faultless pellets. If, on the other hand, the control unit, with the aid of the signal from the detector, discovers a defect in a certain pellet, the control unit will then determine the position of this pellet. The control unit will then activate a separate sorting-out device in the form of an array of compressed air nozzles that are arranged below the conveyor, at the end portion thereof. If a defective pellet is discovered, a corresponding nozzle will be activated, through which a directed air jet is fed towards the pellet in question, just as it passes over the summit at the end of the conveyor. This will in turn entail that the pellet will be blown away somewhat and will land in the second container, which is thus intended for defective pellets.
If all the pellets in a manufacturing process for an insulating cable coating are checked regarding purity, this will consequently correspond to the raw material being cleansed to 100%, which creates conditions for an extremely high purity in the manufacture of an insulation coating for an electrical cable. A further advantage is that the sorted-out and defective pellets can be analysed, providing a possibility for deduction of the reason why they were contaminated.
Even though the above known method generally functions to satisfaction, it has some drawbacks. Primarily it should be noted that it has a limited capacity, i.e. it can only feed forward and analyse a certain, limited amount of pellet per unit of time.
Another disadvantage of the known system relates to the fact that the pellets passing over the top at the end of the conveyor without being sorted out, i.e. without being affected in the above discussed manner by an air jet, will follow a certain track or trajectory before falling down into the first container. Within this trajectory there is a certain scattering, entailing that a certain amount of faultless pellets will pass the first container in spite of being faultless, and will instead be falling into the second container.
Thus, a requirement exists for devices and methods for sorting of pellets, providing an increased capacity and an improved efficiency, i.e. a reduction of the amount of pellets that are sorted out in spite of being faultless.
The object of the present invention is to provide an improved device for sorting of pellets, which provides an increased capacity and efficiency. This object is achieved with a device, the characteristics of which are described in the appended claim 1. The invention also relates to a method for sorting pellets, the characteristics of which are described in the appended claim 7.
The invention comprises a transportation device for feeding the pellet, a first container for faultless pellets fed over the end portion of the transportation device, a second container for defective pellets, a detector for detecting of defective pellets and a sorting device for feeding the defective pellets to said second container. The invention is characterised in that the transportation device is arranged with an angle of inclination relative to the horizontal plane which is selected within a predetermined interval corresponding to a predetermined, limited scattering of the trajectory of the faultless pellets fed over the end portion of the transportation device.
According to a preferred embodiment of the invention, the transportation device consists of a vibration feeder. Furthermore, the invention preferably comprises a frequency sensor for measuring the vibration frequency of said vibration feeder, and a control unit functioning to operate the vibration feeder at a frequency adjusted to generally coincide with the mechanical resonance frequency thereof.
Through the invention, an even and controlled flow of pellet, and a high capacity of detection and sorting out of defective pellets, is achieved. A further advantage is that the sorted-out, contaminated pellets may be analysed at a high capacity, offering a possibility of tracing the reason why they were contaminated.
Advantageous embodiments of the invention are described in the appended, dependent claims.